// Copyright (c) 2003-2004 Brian Wellington (bwelling@xbill.org)

package org.xbill.DNS;

/**
 * Helper functions for doing serial arithmetic. These should be used when
 * setting/checking SOA serial numbers. SOA serial number arithmetic is defined
 * in RFC 1982.
 * 
 * @author Brian Wellington
 */

public final class Serial {

	private static final long MAX32 = 0xFFFFFFFFL;

	private Serial() {
	}

	/**
	 * Compares two numbers using serial arithmetic. The numbers are assumed to
	 * be 32 bit unsigned integers stored in longs.
	 * 
	 * @param serial1
	 *            The first integer
	 * @param serial2
	 *            The second integer
	 * @return 0 if the 2 numbers are equal, a positive number if serial1 is
	 *         greater than serial2, and a negative number if serial2 is greater
	 *         than serial1.
	 * @throws IllegalArgumentException
	 *             serial1 or serial2 is out of range
	 */
	public static int compare(long serial1, long serial2) {
		if (serial1 < 0 || serial1 > MAX32)
			throw new IllegalArgumentException(serial1 + " out of range");
		if (serial2 < 0 || serial2 > MAX32)
			throw new IllegalArgumentException(serial2 + " out of range");
		long diff = serial1 - serial2;
		if (diff >= MAX32)
			diff -= (MAX32 + 1);
		else if (diff < -MAX32)
			diff += (MAX32 + 1);
		return (int) diff;
	}

	/**
	 * Increments a serial number. The number is assumed to be a 32 bit unsigned
	 * integer stored in a long. This basically adds 1 and resets the value to 0
	 * if it is 2^32.
	 * 
	 * @param serial
	 *            The serial number
	 * @return The incremented serial number
	 * @throws IllegalArgumentException
	 *             serial is out of range
	 */
	public static long increment(long serial) {
		if (serial < 0 || serial > MAX32)
			throw new IllegalArgumentException(serial + " out of range");
		if (serial == MAX32)
			return 0;
		return serial + 1;
	}

}
